1. Field of the Invention
The present invention relates to an image output apparatus that inputs image data of a multi-tone image at a preset input resolution, converts the input image data to generate dot creation data, and creates dots according to the generated dot creation data on a printing medium at an output resolution of not lower than the preset input resolution.
2. Description of the Related Art
The printing resolution of printers and other printing apparatuses has been increasing year after year and often exceeds the resolution of images taken with digital cameras. In such cases, one pixel of image data to be printed corresponds to a set of multiple pixels as printing units. For example, when the resolution of an object image to be printed is 360 dpi×360 dpi and the printing resolution is 1440 dpi×720 dpi, one pixel of the image data corresponds to a set of 4×2 pixels in the printing apparatus.
In such cases, the tone value of each pixel requires multi-valuing according to the number and the positions of dots to be created in a group of 4×2 pixels. The simplest method applies the principle of area tone to each group of 4×2 pixels to express 9 different tones from the state of no dot creation in any pixel to the state of dot creation in all the pixels (density pattern method). The applicant of the invention has proposed a method that locally applies the systematic dither technique to the set of 4×2 pixels to enable the high-speed image processing while keeping the high picture quality by the systematic dither technique (see International Publication No. WO-2004-086750A). This innovative method enables the extremely high-speed halftoning process and ensures the high picture quality equivalent to the picture quality attained by the conventional systematic dither method.
Images including characters or letters generally require the sharper reproducibility at the edges of the characters, compared with natural images. The proposed technique has advantages in the processing speed and the resulting picture quality. The high reproducibility of the edges of the characters requires the high resolution of an input image. The high resolution of the input image, however, may cause undesirable extension of the total processing time by this proposed method, like the conventional methods.
The object of the invention is thus to enhance the processing speed when image data includes edges, such as characters and symbols.